The present invention relates to an alloy of Fe-Mn-Si-Cr, and more particularly to a Fe-base shape memory alloy which has good cold worbability and exhibits an improved shape memory effect by a structure hardening through a heat treatment.
Generally, the shape memory alloy has the properties to return to its original shape with a transformation when it is heated over its critical temperature after deforming its shape at low temperature. Accordingly, the shape memory alloy is utilized in various industrial fields such as piping joints for hydraulic equipments, robots, thermo control elements and the like.
As a typical shape memory alloy, a Ni-Ti shape memory alloy may be given which is in practical use. The Ni-Ti shape memory alloy has the good mechanical properties such as elongation rate, yield strength, tensile strength, toughness and the like, while the elements of Ni and Ti not only have high price but also require vacuum melting in manufacturing. In addition, the Ni-Ti shape memory has also a problem of not being used in various fields since the room temperature working such as mechanical working and elongation are difficult.
It has, therefore, been developed a copper-base shape memory alloy which is available with low cost, to substitute the Ni-Ti shape memory alloy, but it is inferior to existing Ni-Ti shape memory alloy in its mechanical properties such as strength and elongation rate, and the thermal stability. It has also problems of low elongation and aging effect due to the grain being coarse.
Japanese Patent Publication No. Sho 61-201761 discloses a Fe-base shape memory consisting of 20-40% of Mn, 3.5-8% of Si and small quantities of Cr, Ni, Co, Mo, C, Al, Cu and the balance being Fe. The above mentioned Fe-Mn-Si base alloy is known that it exhibits an improved shape memory effect by a small quantity of additives and its manufacturing process is simple and it also exhibits good strength and toughness.
Moreover, the highest applied temperature of this Fe-base shape memory is 300.degree. C. while that of the conventional Ti-Ni shape memory alloy is about 150.degree. C., therefore it exhibits a good thermal stability in practical use.
Particularly, it is known that the Fe-Mn-Si alloy exhibits the best shape memory effect in the range of 30-32% of Mn and 6% of Si. However, in such a composition rate the cold working is almost impossible on account of the excess Si, thereby giving rise to some problems that manufacturing of plate or wire is difficult and the work hardening required to improve the shape memory effect is not obtained.